Process-Based Modelling of the Brent Delta

Abstract

The stratigraphic and tectonic framework of the Brent Group and their respective depositional environments is well studied and documented in the literature. The lower progradational Brent deltaic sequence is the response of a high sediment supply rate outpacing the accommodation space rate caused by subsidence and an overall sea level rise stage. These types of systems are typically overfilled basin systems and their facies distribution and stratigraphy are considered be governed by autogenic processes. This implies that the main (fluvial, climatic and basinal) forcing conditions were steady making these formative processes greatly dependent on the geometrical characteristics of the receiving basin and the river delta. While wells and seismic data give a good overview of the stratigraphic sequences, there is still insufficient understanding of the detailed stratigraphic characteristics and facies development within the Brent’s deltaic sequences as response to steady forcing and specific-local scale conditions. This research will focus on the effect of the antecedent bathymetry over which the delta prograded. This antecedent bathymetry is formed by the top of the Oseberg Formation, a coarse grained fan delta. The research objective of this MSc. project is: How the geomorphological evolution, sediment partitioning, facies evolution and stratigraphic response are affected under variable bathymetric conditions by (1) changes in the shoreline direction with respect to the Oseberg´s Fm. pinch out and (2) variable substrate erodability. To address the research objective, a downscaled Brent fluviodeltaic model was created for the Huldra area accounting for the bathymetry generated by the Oseberg Fm. pinch out and also the presumed steady forcing conditions. Published data and data available from STATOIL ASA were used to constrain the basinal and fluvial forcing conditions and the initial bathymetry prevailing in the studied area during Early Jurassic times. Numerical experiments using a hydrodynamic model (Delft3D) show that progradation over the pinch out (shelf edge) can occur under steady sea level condition, producing lobate geometries and basinward sediment sand prone wedge accumulation reaching up to 25 m of thickness. Furthermore they also shows that at, for the anticipated wave climate, no wave induced reworking or transporting of the sand-size grain sediments occurred below 5m water depth because the near-wave orbital velocities are below the threshold transport value. Changing shoreline orientation impacts the wave spreading angle and thereby the affects the interplay between river and basinal forcings. These influence the near coastal reworking pattern as well upstream characteristics of the prograding delta (such as channel avulsion, shifting and abandonment). The numerical experiments including the erodability of the substrate show a large impact on the evolution of the delta. It controls the geometry of the distributary channels. A non-erodible substrate generates wide channels while an erodible substrate produces deeper yet narrower channels since incisions can occur. In addition, it provides a variable but limited sediment source to the delta and the shelf coming from the eroding river bed. While Delft3D is a hydrodynamic model generally applied for engineering scale problems, this study shows it can also be applied to a more geological context. In this aspect we were able to produce new and more geological visualizations not produced before with Delft3D including chronostratigraphic bed level changes, facies model, wheeler diagrams, and reservoir geology maps.